Role of surface heterogeneity in the removal of ammonia from air on micro/mesoporous activated carbons modified with molybdenum and tungtsen oxides

Micro/mesoporous wood-based activated carbons were impregnated with ammonium metatungstate or ammonium molybdate and then heated at 500 °C in inert atmosphere to impose conversion of impregnants to oxides. On the materials obtained, adsorption of ammonia from moist air without and with prehumidification of carbon bed was carried out, followed by desorption measurements. The initial and exhausted samples were characterized by FT-IR, potentiometric titration, thermal analysis and adsorption of nitrogen. The results suggested that ammonia is mainly adsorbed on oxide centers. The presence of water facilitates the process via involvement of Brønsted centers leading to the formation of ammonium salts. Besides, preadsorbed water contributes to adsorption of ammonia via dissolution in the water adsorbed in pores. This ammonia can be considered as weakly adsorbed since it is easily removed when the carbons are purged with dry air. Existence of high degree of heterogeneity in pore sizes causes that some impregnates accumulate in small pores and thus their centers are inefficiently used for ammonia retention.